Difference Between Planet and Moon

Celestial objects are curious things. From the very first days of civilization, human have wondered about the secrets and the majesty of the night sky. Captured in their imagination, these celestial objects were given life in the form of deities. These mysterious objects were an important part of their understanding of the universe and believed to influence their lives in many ways.

As science developed, study of these celestial wonders became more logical and understanding about the planets changed completely. Study of their motion gave rise to new scientific theories, and some theories were validated using these observations.

Planet

Planet is an astronomical body orbiting around a star, which has taken a spherical or nearly spherical shape under its own gravity and has a stable clear orbit.

Planets were known to men from the ancient times. Knowledge of their presence can be found in almost every ancient civilization of the world. In many societies, these fascinating objects in the sky are considered to be divine, and their knowledge on these depended, basically, on the naked eye observation.

Greek civilization supported intellectual discovery into many fields, and astronomy was one of them. Their observations led them to call these unusual objects compared to stars as wanderers. This name was given because, relative to the background stars, these moved forward, westward, sometimes eastward in the night sky. Therefore, they were considered separate from the rest of the stars.

In the ancient world, the existence of 7 planets only was known. Ordered according to the increasing distance from the sun, they are Mercury, Venus, Earth, Mars, Jupiter and Saturn. These were named after the deities of the mount Olympus in Greek mythology.

The discovery of telescope led to the discovery of more planets and, taken together with the sun, it is called the solar system. According to modern understanding there are 8 planets in the solar system, with Uranus and Neptune being the last two. First four planets of the solar system are terrestrial planets with a solid surface visible from outer space. Each of these planets has atmospheres but very different from each other. The outer four planets are known as the Jovian planets or the gas giants. These planets are made of primarily gases, therefore, has enormous atmospheres. Mercury is the smallest planet, and Jupiter is the largest planet.

Discovered in 1930 by Clyde Tombaugh, the Pluto was considered as the outermost planet of the solar system. But the definition brought forward by the International Astronomical Union (IAU) in 2006 caused the demotion of the Pluto to a dwarf planet. Conditions for an astronomical object to be considered as a planet are given below.

1. The object orbit around the sun, or a star or a stellar remnant

2. The object is in hydrostatic equilibrium

3. The object has cleared the neighborhood of the orbit and dominant in the vicinity of the orbit.

Any object that satisfies first and the second condition but violates the third is known as a dwarf planet. The orbit of Pluto is heavily influenced by the gravity of Neptune; therefore, not considered as dominant and has cleared the neighborhood of the orbit. There are 5 known dwarf planets in the solar system. Those are Ceres, Pluto, Haumea, Makemake and Eris.

Planets beyond our solar system also have been discovered. The advancement of observation technologies has led to this discovery by direct observation or deductions from indirect evidence. Planets orbiting the stars other than sun are generally called extrasolar planets or “Exoplanets”. Planets, varying from many times larger than Jupiter to small as earth, have been discovered, but the small ones may exist without being discovered because of the size.

Moon

Moon is a natural satellite orbiting around a planet. Our planet has a natural satellite and it is called “moon”. But the term has evolved into a give broader sense, and it can be used to refer to any natural satellite orbiting a planet.

The first moons to be observed apart from the moon of the earth are the four Galilean satellites Io, Europa, Ganymede, and Callisto of the Jupiter system. Ganymede is the largest moon in the solar system, and Jupiter has 67 moons and Saturn have 62 moons. Mars has two moons; Phobos and Deimos. Uranus has 27 planets, and Neptune has 13 moons. The dwarf planet Pluto has 5 confirmed moons, and Haumea has 2.

Some of the large moons are terrestrial, i.e. made of rocky or metallic material. Some of the moons are made plainly of ice while some are made of a mixture of ice and rock. Several moons in the solar system have interesting characteristics. Lo, the closest moon to Jupiter has the highest volcanic activity anywhere in the solar system, as a result of the strong tidal forces acting in the interior of the moon. Europa is an icy moon. The surface is covered with ice while the interior is in liquid form as a result of the temperature generated by the tidal forces.